Heretofore, power semiconductor devices such as diodes, thyristors, triacs, GTO (Gate Turn Off) thyristors, bipolar transistors, MOS-FETs, and IGBTs (Insulated Gate Bipolar Transistors) are used in various power supply apparatuses. In these power semiconductor devices, a main current that flows through the power semiconductor device is controlled by switching control or analog control. These power semiconductor devices are devices serving as nuclei for implementing stabilized power supply apparatuses, such as switching regulators and linear regulators, and inverters for performing conversion to power having an arbitrary frequency and an arbitrary output voltage.
In these power semiconductor devices, there are switching loss caused by superposition of transient voltage and current at the time of switching and conduction loss caused at the time of conduction. These losses are converted mainly to heat. By the way, the conduction loss has such a characteristic as to become small as the on-resistance is decreased. This on-resistance corresponds to composite resistance of the channel resistance, bulk resistance, and so on existing within the semiconductor except contact resistance between electrodes and semiconductor layer interfaces in the power semiconductor device. Heat generated by the power semiconductor device causes a temperature rise of the power semiconductor device itself. Due to this temperature rise, the power semiconductor device operates at high temperature. Due to this high temperature operation, heat generation of the power semiconductor device is promoted. Such positive feedback is caused. As a result, thermal destruction of the power semiconductor device is caused by thermal runaway.
Usually in the power supply apparatus, therefore, the power semiconductor device itself is provided with a radiation mechanism and in addition, with a radiator such as radiation fins for radiating heat generated by the power semiconductor device. Furthermore, a radiation fan is provided in order to improve the radiation effect in some cases. Furthermore, there is provided a fail safe mechanism that senses the temperature of the power semiconductor device and stops operation of the power semiconductor device when the temperature rises to such a value as to cause thermal runaway.
However, the radiator is formed of a good thermal conductive material, such as aluminum, in order to provide the radiator with a function of heat sink as well. This results in a problem that the whole power supply apparatus becomes large in weight and capacity. Especially as for mobile power supply apparatuses for vehicles or portable power supply apparatuses, emergence of power supply apparatuses reduced in size and weight is demanded strongly.
For example, in a conventional power supply apparatus shown in FIG. 16, a radiator 302 having a large weight and a large capacity is needed. The power supply apparatus shown in FIG. 16 is a DC—DC converter power supply apparatus of a vehicle. A MOS-FET using a Si semiconductor material is incorporated in the power supply apparatus as a switching element. The apparatus main body 301 encloses all the elements forming the power supply apparatus. On the top of the apparatus main body 301, a radiator 302 formed of aluminum is provided. On a junction interface between the radiator 302 and the apparatus main body 301, a MOS-FET, which is not illustrated, stick to the radiator 302. Heat generated by the MOS-FET is absorbed by the radiator 302, and radiated by fins disposed on the top of the radiator 302. Because of installation of the radiator 302, the weight and volume of the whole power supply apparatus become excessively large.
Furthermore, the radiator needs to stick to the power semiconductor device in order to favorably transfer the heat from the power semiconductor device. This brings about limitation on design that the radiator needs to be disposed with due regard to the outer periphery of the casing of the power supply apparatus and the radiation path. This results in a problem that the degree of freedom of the power supply apparatus is reduced. In addition, as for devices such as vehicles using the power supply apparatus, design of the whole device must be changed according to the disposition position of the power supply apparatus. Thus, there is also a problem that the design of the whole device is largely affected.
In addition, in designing the radiator, it is necessary to conduct sufficient radiation design with due regard to the ambient environment of the power supply apparatus. In addition, it is necessary to prevent the power semiconductor device, which is a heat source, from affecting other circuit elements having low heat-resisting property. This results in a problem that much time and labor are required for radiation design and arrangement design of other circuit elements included in the power supply apparatus.
Furthermore, a heat protection circuit for preventing thermal runaway of the power supply apparatus is needed. This heat protection circuit monitors the temperature changes of important components, such as the power semiconductor device, included in the power supply apparatus. When the temperature has risen to a predetermined value, the heat protection circuit conducts fail safe control, such as stopping the power supply apparatus and causing shift to a low dissipation mode. This heat protection circuit is a complicated circuit that senses the temperature, output current, and so on and conducts a shift to fail safe control by using a logic processing circuit. Thus, there is a problem that the power supply apparatus must have a heat protection circuit having such a complicated circuit.
Recently as semiconductor devices having high heat-resisting property, high breakdown voltage, high operation rate and low conduction loss, GaN (galliumnitride) FETs (Field Effect Transistors) have been developed.
Heretofore, such power supply circuits have been applied to, for example, automobiles, various public welfare devices (such as video, television and audio devices), and industrial devices (such as personal computers, communication devices, and FA control devices).
The above described power supply circuit includes a transformer. A transistor made of, for example, a power MOS element turns on and off according to a gate signal. As a result, an output voltage is generated on a secondary winding side.
In the above described power supply circuit, however, the power MOS element used as the transistor, such as a power MOS-FET (2SK2313) generates much heat. Therefore, it is necessary to perform the radiation design accurately. A channel temperature Tch max of the power MOS-FET itself at an ambient temperature of 85° C. is calculated as                               Tch          ⁢                                           ⁢          max                =                              Ta            ⁢                                                   ⁢            max                    +                      Ptotal            ×                          Rth              ⁡                              (                                  ch                  -                  a                                )                                                                                      =                      85            ⁢            °            ⁢                                                   ⁢                          C              .                              +                2                                      ⁢            W            ×            50            ⁢            °            ⁢                                                   ⁢                                          C                .                            /              W                                      ⁢                                                           =                  185          ⁢          °          ⁢                                           ⁢                      C            .                              where Ta max: ambient temperature
Ptotal: total loss
Rth (ch-a): thermal resistance between channel and environment.The temperature rises up to the channel temperature or higher. Therefore, it is necessary to provide a radiation plate. Supposing derating of 50° C. for a channel temperature of 150° C., the radiation plate design is represented as       θ    ⁢                   ⁢    f    <            θ      ⁢                           ⁢      ch        -    a    -                  (                              θ            ⁢                                                   ⁢            I                    +                      (                                          θ                ⁢                                                                   ⁢                c                            +                              θ                ⁢                                                                   ⁢                s                                      )                          )            ⁢                                                  =                            ⁢                                                7.5                  ⁢                  °                  ⁢                                                                           ⁢                                                            C                      .                                        /                    W                                                  -                                                                                                      ⁢                              (                                                      0.833                    ⁢                    °                    ⁢                                                                                   ⁢                                                                  C                        .                                            /                      W                                                        +                                                                           ⁢                                      0.8                    ⁢                    °                    ⁢                                                                                   ⁢                                                                  C                        .                                            /                      W                                                                      )                                                                                        =                            ⁢                              5.9                ⁢                °                ⁢                                                                   ⁢                                                      C                    .                                    /                  W                                                                        where θf: thermal resistance of radiator
θch-a: total thermal resistance between channel and environment
θi: thermal resistance between junction portion and case (internal thermal resistance)
θc+θs: thermal resistance between case and radiator From the foregoing description, it is necessary to select a radiator having a thermal resistance of 5.9° C./W or less. For example, therefore, a radiation plate made of an aluminum plate of 100 cm2 having a thickness of 1 mm becomes necessary. As a result, the conventional power supply circuit has a problem that the circuit configuration becomes large and heavy because of the radiation plate.
Furthermore, heretofore, such a large current load control apparatus is applied to, for example, lighting control of head lamps of automobiles.
in the above described large current load control apparatus, lighting control of a head lamp is conducted by turning on and off a power MOS-FET formed of, for example, an on/off control switching element provided on a power supply line, which connects a battery to the lamp, under the control of a microcomputer.
In this control apparatus, however, the power MOS-FET used as the switching element of on/off control generates much heat. Therefore, it is necessary to perform the radiation design accurately. A channel temperature Tch max of the power MOS-FET is calculated as                                                                         Tch                ⁢                                                                   ⁢                max                            =                            ⁢                                                (                                      Ta                    ⁢                                                                                   ⁢                    max                                    )                                +                                                      (                                          Ron                      ⁢                                                                                           ⁢                      max                                        )                                    ×                                      (                                          lo                      ⁢                                                                                           ⁢                      max                                        )                                    ×                                                                                                                      ⁢                                                (                                      lo                    ⁢                                                                                   ⁢                    max                                    )                                ×                                  Rth                  ⁡                                      (                                          ch                      -                      a                                        )                                                                                                                          =                            ⁢                              85                ⁢                °                ⁢                                                                   ⁢                                  C                  .                                      +                    0.013                                                  ⁢                Ω                ×                10                ⁢                A                ×                10                ⁢                A                ×                50                ⁢                °                ⁢                                                                   ⁢                                                      C                    .                                    /                  W                                                                                                        =                            ⁢                              150                ⁢                °                ⁢                                                                   ⁢                                  C                  .                                                                                        (        10        )            where Ta max: ambient temperature                Ron max: on-resistance        lo max: current value        
Rth (ch-a): thermal resistance between channel and environment.The temperature rises up to the channel temperature. Therefore, it is necessary to provide a radiation plate. Supposing derating of 50° C. for a channel temperature of 150° C., the radiation plate design is represented as       θ    ⁢                   ⁢    f    <            θ      ⁢                           ⁢      j        -    a    -                  (                              θ            ⁢                                                   ⁢            I                    +                      (                                          θ                ⁢                                                                   ⁢                c                            +                              θ                ⁢                                                                   ⁢                s                                      )                          )            ⁢                                                  =                            ⁢                                                11.5                  ⁢                  °                  ⁢                                                                           ⁢                                                            C                      .                                        /                    W                                                  -                                                                                                      ⁢                              (                                                      0.833                    ⁢                    °                    ⁢                                                                                   ⁢                                                                  C                        .                                            /                      W                                                        +                                      0.8                    ⁢                    °                    ⁢                                                                                   ⁢                                                                  C                        .                                            /                      W                                                                      )                                                                                        =                            ⁢                              9.9                ⁢                °                ⁢                                                                   ⁢                                                      C                    .                                    /                  W                                                                        where θf: thermal resistance of radiator
θj-a: total thermal resistance between channel junction portion and the outside air
θi: thermal resistance between junction portion and case (internal thermal resistance)
θc+θs: thermal resistance between case and radiator From the foregoing description, it is necessary to select a radiator having a thermal resistance of 9.9° C./W or less. For example, therefore, a radiation plate made of an aluminum plate of 6 cm2 having a thickness of 1 mm and a weight of approximately 10 g becomes necessary. As a result, the conventional large load control apparatus has a problem that the circuit configuration becomes large and heavy because of the radiation plate.
Therefore, it is one object of the present invention is to provide a power supply apparatus capable of implementing reduction of the size and weight, conducting flexibly design including the radiation design, and remarkably reducing the time and labor required for the design.
Furthermore, it is an another object of the present invention is to provide a power supply circuit capable of reducing the heat generated by the transistor, thereby making the radiation plate unnecessary, and implementing reduction of the size and weight of the circuit.
Furthermore, it is still another object of the present invention is to provide a large current load control apparatus capable of reducing the heat generated by the on/off control switching element, thereby making the radiation plate unnecessary, and implementing reduction of the size and weight of the circuit.